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You can manually install GRUB onto a device if it was installed to the wrong place by issuing

sudo grub-install /dev/sdX

Quoted: However if the above doen`t make it possible to boot the OS you can use "Rescue a broken system" at the USB drive's startup menu to install GRUB.

Follow the prompts to start up your system, selecting the hard disk partition when requested. The “Enter rescue mode” screen appears which provides options to mount your system. Your broken system will be mounted and made accessible with a command line interface. You can then use command line operations and editors to fix configuration files. If you need to reinstall the boot loader, you can choose the “Reinstall GRUB boot loader” entry.